1. Field of the Invention
This invention relates to multimode optical fibers (MMFs) and, more particularly, to the design and manufacture of such fibers for broadband applications, including coarse wavelength division multiplexing (CWDM). Designs according to the inventive principles address specific manufacturing problems associated with particular dopants by incorporating flat-zones in the dopant concentration profile.
2. Discussion of the Related Art
A typical MMF includes a relatively high-index core region surrounded by a lower index cladding region, with the two regions configured to support the simultaneous propagation of optical radiation in the core region in a plurality of transverse modes. The base material of MMFs is typically silica glass, with the core region being up-doped with one or more dopants (e.g., Ge, Al, P) that increase its refractive index and the cladding region being either undoped or down-doped with one or more dopants (e.g., F, B) that reduce its refractive index. In some designs, dopants such as F or B may also be added to the core region as long as the net refractive index of the core region is still greater than that of the cladding region.
The choice of a specific dopant (and its concentration profile) in the core and cladding regions may be dictated by design characteristics (e.g., index grading, NA, MFD) or performance issues (e.g., bandwidth), or may dictated by manufacturing/process problems associated with the use of a particular dopant (e.g., P, F).
More specifically, Ge-dopant is commonly used to form a near-parabolic index profile in the core region of a MMF, often referred to as a graded index (GI) MMF. While the Ge-doped index profile in a GI MMF can be optimized to achieve a high bandwidth, the high material dispersion of Ge-doped silica limits the spectral width of the high bandwidth region. It is known that both P- and F-doped silica have much smaller material dispersion relative to Ge-doped silica, and fibers made with P- and/or F-dopants have much wider spectral width than conventional Ge-doped fiber [D. Gloge and E. A. J. Marcatili, “Multimode Theory of Graded-Core Fibers,” BSTJ, vol. 52, no. 9, pp. 1563-1578 (November 1973), which is incorporated herein by reference]. However, it is difficult to introduce a high P-dopant concentration during preform processing because P-doped silica has a high vapor pressure, and a significant fraction of P-dopant is burned off during preform collapse. It is also difficult to maintain a circular preform core containing a high P-concentration because of its much lower viscosity than the surrounding silica substrate tube.
Furthermore, upon exposure to either hydrogen or radiation, fibers containing a high P-concentration have a significantly higher added attenuation, and the added attenuation increases monotonically with the P-dopant concentration. Therefore, it is desirable to limit the P-concentration in the fiber core region.
Fabrication of a broadband GI MMF with information transmission capacity of at least 10 Gb/s over a distance of least 300 in requires precise control of the refractive index profile of the core region. The near-parabolic index profile shape guarantees the “time of flight” propagation delay equalization (low modal dispersion) for all modes of light traveling in the core region. Any deviation from the perfect design shape will lead to a spread in travel times for different modes and will degrade the information carrying capacity of the fiber.
The refractive index profile of the core region is determined by a combined contribution from all core dopants (sometimes referred to as co-dopants). In general, in some broadband GI MMF designs it is desirable to use more than one such dopant, each with a specific concentration profile, to achieve required properties of the GI MMF, to improve glass manufacturability, or both. In any case, the resulting refractive index profile should be as close to an ideal mode-delay equalizing profile as possible and should not have any step discontinuities inside the core radius.
The preforms of GI MMFs can be fabricated from suitable gas/chemical precursors by any one of several glass deposition systems, such as MCVD, OVD, etc, each of which typically uses mass flow controllers (MFCs) to control the chemical flow rates during preform processing. Commercial MFCs have difficulty controlling flow rates accurately at low rates near or below their certified values. When MFCs are set to flow at such low rates, large and unpredictable deviations from the target rates can occur. It is particularly challenging to control F-doping at small refractive index levels. For example, the flow rate of SiF4 or other F-containing precursor gases must be decreased by sixteen times when the target F-index level is reduced only two times, say from Δn==−0.001 to Δn=−0.0005, assuming all other chemical flows are kept constant. Therefore, when 10% of the full scale in the SiF4 MFC is needed to obtain Δn=−0.001 index value for F, the same MFC must be set at only 0.625% (10%/16) of the MFC full scale to reach Δn=−0.0005 index for F. Such small MFC settings will result in significant flow rate errors.
While in theory it may be possible to compensate for continuous and repeatable deviations in the dopant precursor flows from their target flows, random or abrupt “step” changes in flows or full flow interruptions cannot be compensated for and will result in an imperfect core profile. As an example, one possible case when such changes may occur is when an MFC is set to control flows below its minimum certified value. It is therefore desirable to expect such process limitations and to modify fiber designs accordingly to mitigate their impact.